This usually occurs after you just fuck around for a while (usually intentionally) and perform at merely an average, or below average level. Then when you decide its time to "bring the rain", you take it more seriously and dominate whatever it is your doing - playing basketball, doing an exam, playing playstation etc.

Okay, enough of this fucking around, it's time to bring the rain.

Dude, I don't give a fuck about these practice tests, I'll just bring the rain on exam day.